This application relates to the finishing of polycarbonate using a melt process, and in particular to a method for quenching residual catalyst used in the polycarbonate-forming reaction.
Aromatic polycarbonates are useful in a great many applications because of their desirable physical properties, including strength and optical clarity. There are three processes known for the production of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial process and the phosgene-based melt process start with the reaction of phosgene with carbon monoxide. The "no phosgene" melt process was developed to eliminate the use of highly toxic phosgene in the reaction process.
Both types of melt processes make use of a diaryl carbonate such as diphenyl carbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst to form a polycarbonate in accordance with the general reaction shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers. In many cases, however, the presence of residual catalyst has a detrimental effect on the quality of the product, leading to poor color, molecular weight or rheological properties. Residual catalyst may also interact with additives, detracting from their efficacy. Thus, it is desirable to reduce the levels of residual catalyst to minimize these interactions. Such reduction is referred to as "quenching."
Commonly assigned U.S. Pat. No. 5,717,057, which is incorporated herein by reference, discloses the use of sulfur-containing acids and acid derivatives such as esters for neutralization of residual alkaline catalyst. The acid or acid derivative is added directly to the polycarbonate product of the polycondensation reaction during post-reaction extrusion and pelletization. As the amount of liquid quencher is extremely low and the amount of quencher is important for the final properties, it is impossible to feed quencher in pure form on an industrial scale. The use of a carrier is needed to maintain the levels of accuracy required.
Quenching of residual alkaline catalyst has been accomplished commercially using n-butyl p-toluenesulfonic acid (a.k.a. n-butyl tosylate) in a powder carrier. The tosylate acts by direct alkylation of the alkaline catalyst, with concomitant degradation of the tosyl moiety. A small amount of liquid quencher is added to a polycarbonate powder dry blend which also contains other additives such as heat stabilizers, UV absorbers, and color stabilizers. Because of the use of a polycarbonate powder, however, this process has the disadvantage of bringing dust into an otherwise dust-free process. Thus, there is room for improvement in the process of making polycarbonates using acidic quenchers. It is an object of the present invention to provide such improvements.